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06-10-2009, 07:31 AM   #1
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Long time exposure must use NR?

I owned a K20D and K100D. Both of them seems to use NR when shooting with long time exposure e.g when shooting with more than 30 sec, it takes similar time to process the NR
I heard from my frd that his 5D / 5DII will do NR too. But it takes only a few seconds.
Why they have such big difference? How about K7?

06-10-2009, 07:44 AM   #2
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If you want to do NR with long exposures it needs to take at least as long as the exposure. *If* the NR is done by Dark Frame Subtraction that is, as it is in the K20D.

This process takes an additional exposure without light entering the camera to measure the impact of the long exposure on the sensor in the same circumstances as the original exposure. This is important information to allow the NR to remove noise only and not image information. I suspect the NR mentioned by your friend on his 5D(II) is not the same kind of NR...

This Dark Frame Subtraction is a good technique, but the problem is you are required to wait after every exposure, which is not something you may want to do every time...

However, the K-7 will apparently allow you to setup the camera so it doesn't do this after a long exposure. This doesn't mean you shouldn't use it however. But K-7 does allow you to make your own dark frame exposure for noise reduction in PP and just go ahead shooting a series of long exposures without the waiting in between shots. Though this requires a more specialised post processing, it's also a less battery consuming process...

Conclusion: You can setup the K-7 so that you don't have to spend the time waiting, but unless you make your own dark frame and process this in post processing, you will not have the same efficiency of NR.

Wim
06-10-2009, 08:39 AM   #3
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QuoteOriginally posted by yipchunyu Quote
I owned a K20D and K100D. Both of them seems to use NR when shooting with long time exposure e.g when shooting with more than 30 sec, it takes similar time to process the NR
If his camera is taking less time to do to the NR, then it is not using the dark frame subtraction method, which requires the camera to actually take another picture at the same shutter speed (but with the shutter closed). It's not "processing" this that takes time, but actually taking the second exposure.

The K20D always does this dark frame subtraction. On all other cameras - including the K100D - it can be turned off via a menu option.
06-10-2009, 08:48 AM   #4
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QuoteOriginally posted by Marc Sabatella Quote
If his camera is taking less time to do to the NR, then it is not using the dark frame subtraction method, which requires the camera to actually take another picture at the same shutter speed (but with the shutter closed). It's not "processing" this that takes time, but actually taking the second exposure.

The K20D always does this dark frame subtraction. On all other cameras - including the K100D - it can be turned off via a menu option.
K100D can turn it off? what a great news. May be I can test it later. Thx for your sharing.

06-12-2009, 10:42 AM   #5
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QuoteOriginally posted by Marc Sabatella Quote
... which requires the camera to actually take another picture at the same shutter speed (but with the shutter closed).
I'm just curious, but if the shutter is closed, why does it need to match the original shot's shutter speed?

Wouldn't the result (of the second shot) be affected by the temperature of the sensor, which should be relatively hotter than it was during the original shot - say a 45 minute exposure for star tracks. (OK, I probably SHOULD use film for those.)

regards,
-tom
06-12-2009, 02:23 PM   #6
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QuoteOriginally posted by tpeace Quote
I'm just curious, but if the shutter is closed, why does it need to match the original shot's shutter speed?
Because the type of noise that the camera is trying to remove is directly proportional to the time the sensor is collecting data.

QuoteQuote:
Wouldn't the result (of the second shot) be affected by the temperature of the sensor, which should be relatively hotter than it was during the original shot - say a 45 minute exposure for star tracks.
That's a good point, Seems the second dark frame might well show too much noise for that reason. Presumably the firmware deals with that somehow, or else the sensor cools down *really* fast once read (which seems possible at least for "short" long exposures).
06-16-2009, 07:35 AM   #7
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Just checked my friend's 5D II today. It seems to use Dark Frame Subtraction too. However, what's the major difference is that during this process, he can still shoot pictures without waiting the completion of the dark frame subtraction. it's so nice.
Just hope K7 or pentax's dslr can implement similar feature in the future. more, it's nice as if the top panel can show the no of sec of the b-mode.
06-16-2009, 09:24 AM   #8
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QuoteOriginally posted by yipchunyu Quote
K100D can turn it off? what a great news. May be I can test it later. Thx for your sharing.
I can turn it off in my K10d as well.

06-16-2009, 09:34 AM   #9
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QuoteOriginally posted by yipchunyu Quote
Just checked my friend's 5D II today. It seems to use Dark Frame Subtraction too. However, what's the major difference is that during this process, he can still shoot pictures without waiting the completion of the dark frame subtraction. it's so nice.
Are you sure? That seems flat out impossible. Most likely, he just has the dark frame subtraction turned off.
06-16-2009, 06:24 PM   #10
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QuoteOriginally posted by Marc Sabatella Quote
That seems flat out impossible.
Not impossible. Had I designed the feature, I would have cached the darkframe when subtracting it (tagged by exposure time, to be invalidated after a given period of time). So, the next exposure would be without any wait
06-17-2009, 01:12 PM   #11
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Oh sure, if you don't do a dark frame per image. That's certainly what some people have asked for regarding the K20D - the ability to use an earlier dark frame (either in camera or in PP). But he specifically claimed it was allowing pictures to be taken *while* doing the dark frame. No way, unless there are two separate sensors involved, and we know there aren't in this case.

And in any case, a quick google didn't turn up references to the 5DII doing anything like this. I think he just had it turned off.
06-17-2009, 01:24 PM   #12
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Just FYI, dark frame subtraction is rather pointless anyway if you shoot RAW.
Modern RAW converters like Lightroom eliminate hot pixels automatically, without needing a dark frame. I have DFS turned off on my K200D and sometimes I see hotpixels in the preview JPG but they automatically disappear when Lightroom has rendered the RAW data.

Last edited by patrickhh; 06-17-2009 at 01:34 PM.
06-17-2009, 01:53 PM   #13
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Dark frame subtraction is not about hot pixels. It's about noise. True, PP programs can remove noise too, but the specific method use in dark frame subtraction is quite effective for the type of noise it removes, and it is *not* available in Pp (unless you happen to have an appropriate dark frame you can use, and the program supports it - most probably don't).
06-18-2009, 02:52 AM   #14
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QuoteOriginally posted by Marc Sabatella Quote
Are you sure? That seems flat out impossible. Most likely, he just has the dark frame subtraction turned off.
haha, I will retry it ASAP and reconfirm this.
But, I guess it's true.
06-18-2009, 04:29 AM   #15
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I posted the question to dpreview.
Someone tested and it really works.

5D MKII owner, pls help to test it.: Canon EOS-1D / 1Ds / 5D Forum: Digital Photography Review
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